Bones and bony structures are susceptible to a variety of weaknesses that can affect their ability to provide support and structure. Weaknesses in bony structures may have many causes, including degenerative diseases, tumors, fractures, and dislocations. Advances in medicine and engineering have provided doctors with a plurality of devices and techniques for alleviating or curing these weaknesses.
The cervical spine has presented the most challenges for doctors, partially due to the small size of the vertebrae and the spacing between adjacent vertebrae. Typically, weaknesses in the cervical spine are corrected by using devices that fuse one or more vertebrae together. Common devices involve plate systems that align and maintain adjacent cervical vertebrae in a desired position, with a desired spacing.
These devices, commonly referred to as bone fixation plating systems, typically include one or more plates and screws for aligning and holding vertebrae in a fixed position with respect to one another. Initial devices used stainless steel plates and screws. In order to remain fixed in place, the screws were required to pass completely through the vertebrae and into the spinal canal. These devices caused many complications and involved significant risks. To allow a screw to pass, drilling and then tapping of the vertebrae was required. In the process, instruments came within close proximity of the spinal cord, which required extreme care on the part of the surgeon.
In addition to the risks of surgically applying bone fixation plates, other complications arose. Commonly, these problems involve loosening and failure of the hardware. Two common failures are the breakage of the plates, and the backing out of the screws into soft tissues of the patient's body. The backing out of the screws is typically a result of the screw's failure to achieve a sufficient purchase in the bone, although the stripping of the screws has also been known to cause this problem. Regardless of the cause of the hardware failures, a surgeon must repair or replace the broken parts, which requires undesirable invasive procedures.
Advances in material science allowed engineers to manufacture bone fixation plates out of materials that would resist breakdown within a body. However, the backing out of screws remained a problem. Many solutions were devised in an attempt to prevent this from occurring. One prevalent solution involved minimizing the length of the screw in order to prevent screw to plate junction breakage of the screw. However, the shortened screw is typically unable to achieve a sufficient purchase in the bone. Shortened screws often provide very little holding power and inadequate tactile feedback to the surgeon. Tactile feedback to the surgeon is important to signal completion of tightening prior to stripping of the screw within the bone.
An alternate solution involves increasing the length of the screws in order to achieve sufficient purchase to hold the plate in place. While the use of longer screws can provide bicortical fixation, this method also has its drawbacks. Primarily, long screws increase the chances of interference with each other when they are screwed into bony tissue at an angle. In addition, many bone fixation plating systems place bone grafts between vertebrae. The bone grafts are eventually supposed to spur the growth of bone between the vertebrae, so that the vertebrae become fused together naturally.
In order for this to occur, the bone fixation plating needs to maintain a desired spacing between the vertebrae, which is filled by the bone grafts. However, it is common for the bone grafts to experience compression, which separates at least one of the adjacent vertebrae from the bone graft. Cervical plates that employ long screws do not allow for sufficient movement of the vertebrae to accommodate the compression of the bone graft, because the purchase of the screws is too great. Thus, the vertebrae cannot move and are unable to adjusting to the compression of the bone graft.
Another method of preventing the backing out of screws involves placing a second plate over the screws. This second plate functions to interlock the screws, preventing them from backing out. However, this method of securing screws often becomes bulky, resulting in a large and undesirable profile. In addition, these configurations require carrying out multiple steps or using a multi-piece assembly in order to block an opening through which a loose fastener head may pass. For instance, the use of a c-ring that can expand as the fastener head is inserted requires additional components and assembly time to form a plate. Moreover, multi-component designs may lose their ability to retain a fastener over time due to material failure, relaxation, or the like. Additionally, multi-component configurations may not provide sufficient ability to lag the plate to the vertebral body.
One additional drawback of many designs is that they add to the overall height of the plate. It is desirable to maintain a low profile design for many reasons, such as to minimize irritation to surrounding tissue. For example a plate design having a high overall height or a receptacle design that does not prevent screw backout may cause a patient to suffer from dysphasia. Ultimately, the screw or plate may irritate or wear through neighboring tissue. In addition, a high height plate or unretained loose screw in the lumbar spine may be abrasive to the aorta or vena cava. Severe abrasion by the plate or screw in this instance may puncture the aorta or vena cava and cause internal bleeding.
In addition, many of these plates were not designed to allow for the locking of all of the screws, which left some of the screws susceptible to backout caused by tiny vibrations, or micromotion. Some methods attempted to reduce the profile of the total system by using small parts. However, this led to the small parts falling off and getting lost. In addition, the smaller parts are fragile and require special instruments in order to insert or manipulate them. In addition, because of their small size, incorrect placement relative to the axis of the plate often causes sharp and jagged shavings to be formed as a locking screw contacts an improperly seated bone screw.
Prior attempts at increasing the screw purchase have resulted in risky procedures, or an insufficient ability to adapt to movement. Attempts and decreasing the profile of bone fixation plates have resulted in lost parts, or insufficient purchase. A continuing need exists for an apparatus that is able to quickly and reliably lock a plurality of screws into place while maintaining a low profile.